A new approach to electrical heating appliances in recent years has been self-regulating heating systems which utilize materials certain types of PTC (positive temperature coefficient) of resistance characteristics. The distinguishing feature of such materials is that upon attaining a certain temperature, a substantial increase in resistance occurs, an increase that for many compositions effectively precludes them from drawing any significant current. Heaters known to the prior art utilizing PTC materials generally exhibit, therefore, a relatively small increase in resistance with increasing temperature change as heating is initiated. However, at some elevated temperature, the resistance begins to increase rapidly with further temperature increase. The temperature (which may be a temperature range) at which the rapid increase in resistance begins to often designated the switching or anomaly temperature (T.sub.s). Above T.sub.s, resistance can become so high that the current is in effect switched off. Therefore, in actual practice the T.sub.s temperature represents about the maximum temperature to which the PTC heater element will rise. In many applications this has significant advantages over other means of temperature regulation, such as thermostats, fuses or in line resistors, since it eliminates the need for elements that, on a relative basis, can be costly, require added space, be prone to failure or have other shortcomings.
Many well known PTC materials are ceramic in nature. They have numerous applications but their rigidity precludes their use in other instances. However, it is also known that certain electrically conductive polymer compositions exhibit PTC behaviour. Such materials generally comprise one or more conductive fillers such as carbon black or powdered metal dispersed in a crystalline thermoplastic polymer. The most useful types of PTC composition are prepared from highly crystalline polymers and usually exhibit a distinctive rise in resistance a few degrees below the crystalline melting point of the polymer. Accordingly, the T.sub.s of such compositions will be at or near the crystalline melting point of such polymers. A graphical representation of the effect of increasing temperature on resistance for a typical polymeric PTC composition and a time-temperature curve are shown in FIGS. 13 and 14.
There are many applications in which heating elements exhibiting typical PTC character as exemplified are adequate. However, under other circumstances it is desirable that, temporarily at least, the heater element exceed the T.sub.s temperature before the PTC composition exerts its controlling influence to fix the temperature of the heater at T.sub.s. In the past, it has been proposed that transient and localized heating of a substrate above the anomaly temperature of a PTC element could be achieved by immersing in the substrate a second heating element connected in series to the PTC element but thermally isolated therefrom. See U.S. Pat. Nos. 3,375,774 and 3,551,644. In these references, the second heating element is selected to have a higher initial resistance than the PTC element. Therefore, when connected to a source of electrical current, the second element heats first and heats the adjacent substrate which may be, for example, the water in a coffee pot. The heated substrate acts as a medium of heat transfer to warm the PTC layer to its anomaly temperature. The temperature stabilizes at this temperture. Many PTC compositions, inasmuch as they are crystalline thermoplastic polymers, if crosslinked, as by ionizing radiation or by chemical means, can be rendered heat recoverable by being deformed above their crystalline m.p. and allowed to cool while deformed. Compositions suitable for use in heat recoverable articles and the methods by which they are obtained, are disclosed, for example, in Cook, U.S. Pat. No. 3,086,242, the disclosure of which is incorporated by reference.
As is now well known, heat recoverable polymeric articles like those disclosed in the Cook patent undergo recovery from their heat recoverable configuration upon being heated without restraint above their crystalline melting point. Most efficient recovery occurs when the temperature of the polymer is well above, for example at least about 10.degree. C above, the crystalline melting point. Typically, the recovery of heat recoverable articles is effected by heating the article with a torch or other open flame.
A frequent application of polymeric heat recoverable articles is as protective coverings about substrates, for example, elongate objects such as pine or electrical cable, where a splice has been made. One method by which this can be done is to install a tube of heat recoverable material capable of recovering to a smaller diameter over the substrate, heating it to achieve recovery. In most applications this heat is supplied by an open flame as described above.
However, as will be apparent to those skilled in the art, there are many applications in which the use of an open flame is unacceptable, for example, in mines or other relatively close places wherein explosive gases can be present, where space limitations are critical or where the protected article is delicate.
In view of the foregoing, as might be expected, it has been proposed to employ electrically self-heating, heat recoverable articles in such instances. This can be done by using conductive polymeric compositions like those previously described in the recoverable member. However, if the recoverable article comprises a conductive polymer having PTC properties, the temperature will only reach T.sub.s, which may be too low to bring about rapid and/or complete recovery. Of course a non-PTC conductive polymer could be used. However, though such a composition may be safely employed where open flames are dangerous it can still overheat and damage itself or the delicate components it encapsulates unless closely monitored by a workman or unless precautions such as thermostats or fuses are employed. When these additions are required, many of the advantages of a self-heating article are no longer present.
One solution to this problem has been to employ in heating elements compositions that continue to exhibit PTC behavior above the crystalline melting point. Compositions exhibiting such behaviour, including those comprising a crosslinked blend of an elastomer and a thermoplastic are described in Horsma et al., "Positive Temperature Coefficient of Resistance Compositions,"Ser. No. 601,639, having the same assignee as the present invention, the disclosure of which is incorporated by reference. Though valuable in many applications such compositions are not suited for all purposes.
Accordingly, it is an object of this invention to provide improved self heating articles that are self-regulating.
It is another object of this invention to provide novel heaters regulated by PTC compositions that transiently exceed the temperature controlled by the PTC composition.
Yet another object of this invention is to provide a self heating heat recoverable article that is self-regulating.